Interspecies platelet immunizations in marmosets lead to formation of antibody to the donor platelets, deposition of IgG on the host's platelets and thrombocytopenia. The clinical and physiologic manifestations of this disease closely resemble that observed in posttransfusion purpura in man, a hematologic disorder falling within the syndrome of idiopathic thrombocytopenic purpura. In the marmoset, however, the form of platelet (antigen) administration and route of injection are important variables since platelets suspended in saline and inoculated intravenously are without effect while intramuscular injections frequently lead to the disease syndrome. Preliminary studies indicate this differentiation in response to route of administration may be due to either the nature (molecular species) or titer of antibody formed. In a few animals, antibody toward autologous or host-type platelets have also been found, but these are detectable only when the animal is severely thrombocytopenic. Animals recovering from the induced thrombocytopenia can be maintained with IgG ion platelets by regular inoculations of the donor platelet antigen in the absence of any thrombocytopenia. Platelet turnover studies have shown an increased random destruction of platelets in such animals, suggesting a compensatory mechanism operating to maintain normal platelet levels. Quantitation of IgG on the platelets following induction of the disease has revealed amounts comparable to that found in patients with thrombocytopenia that is also immunologically mediated.